Plasma generator and accelerator



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Sept. 29, 1964 E. B. MAYFIELD ETAL 3,150,433

Pusmcaum'ron AND Accswmron nud lay 1o. 1962 F l G. I

h- POWER l SUPPLY HEAVY FUEL Emi B. MAYFIELD VERNAL JOSEPHSON INVENTORS.

ATTORNEY.

United States Patent O 3,150,483 PLASMA GENERATOR AND ACCELERATOR EarleB. Mayfield, Torrance, and Vernal Josephson, Palos The present inventionrelates to the magnetohydrodynamic art and more particularly to ahigh-specific impulse engine suitable for space operation.

Magnetohydrodynamic prior art includes several proposed ionic thrustdevices arranged to accelerate ions to high velocities suitable forspace vehicle propulsion. Although, with presently available powersupplies, the thrust of such engines is relatively small, theirefficiency is extremely high in proportion to the fuel mass consumed andthe thrust is suicient to accelerate vehicles from orbital to deep spacevelocities. Specific impulses of the order of 10,000 seconds arecontemplated. In one type of prior art arrangement, specific-impulseengines have admitted thereto ionizable or ionized material throughnumerous apertures around the periphery of the engine chamber. Thisapproach results in several problems including non-uniformity of theejected mass whereby the thrust obtainable is relatively undeterminable,and the overall system has less than maximum thrust and efficiency.

It is, therefore, an object of our invention to provide a new andimproved specific-impulse space engine.

According to one embodiment of the present invention a light-gaspropulsion fuel is admitted into a thrust chamber continuously from acentral porous emitter.` The emitter is arranged to allow the gas todiffuse through a central electrode into the engine chamber, as soon asthe density within the engine chamber reaches approximately 101particles per cubic centimeter the system is ionized and pulsed (orswept) by a magnetic field in the manner of operation of wavepropagation through a coaxial cable. The speed of the sweeping magneticfield is of the order of 105 meters per second, and, when usinglight-gases such as hydrogen, or deuterium, substantially all of theions ow from the engine at this velocity.

The subject matter which is regarded as this invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention, however, as to its organization andoperation, together with further objects and advantages thereof, willbest be understood by reference to the following description taken inconnection with the accompanying drawing in which:

FIG. 1 is `a partially schematic side sectional view of an enginecomponent illustrating one embodiment of the present invention;

FIG. 2 is a modiiication of the embodiment shown in FIG. 1; and

FIG. 3 is a schematic view of an engine utilizing several of the enginecomponents of FIG. 1.

Referring now to the drawing wherein like numbers refer to similarparts, in FIG. 1 a cylindrical engine chamber or driving section issubstantially enclosed around its lateral surface and one end by ahousing 10.

A power supply 11 energizes a field or pulse coil 12 to generate asweeping magnetic field 13 that passes from the closed end to the openend of the housing 10 to ionize and sweep any gas particles 14 throughthe engine exhaust port 16. The selection of the most appropriate modeof operation of the sweeping lield 13 depends on many considerationsknown in the art of wave guides and coaxial cables. One preferable modethat can be utilized is a TM wave-which is a transverse magnetic lCCwave having no component of its magnetic force in the direction oftranslation of the wave along the wave guide. In order that the sweepingfield 13 be most effective, the frequency of sweep is a function of therate at which a leak tube 18 is lled.

A major problem in this particular art is the attaining of a desireduniform gas density within the housing 10. As shown, a gas diffusingmeans illustrated as the hollow cylindrical palladium or nickel leaktube 18 passes through the center of the housing 10 and receives a lightionizable gas such as hydrogen or deuterium from a gas supply tank 19 atits input end 20. In order to allow the gas to diiuse into thepropulsion chamber, the leak tube 18 is maintained at an elevatedtemperature of the order of 400 C. As illustrated, the leak tube 18 isheated by a convenient means such as a coiled heater filament 22.

Additionally, the wall of the tube 18 can be partially lled with ferriteor doped with ferrite materials both to improve the configuration of thesweeping field 13 and to allow inductive heating of the leak tube 18. Asis well known, the addition of a ferro-magnetic material to thepalladium or nickel tube greatly increases its permeability so that thewave form across the chamber towards the center electrode is more nearlyperpendicular to the electrode. In fact, it is preferred that the power`operating mode of the engine be so predetermined that a major portion ofthe heating, after initial energization be accomplished by the energy ofthe sweeping magnetic field 13. Usually a leak tube 18 of this type hasa wall thickness of the order of ten mils.

In a system wherein the inner diameter of the chamber within the housing10 is of the order of ten centimeters, the leak tube 18 has a diameterof the order of one centimeter. Thus the surface area of the leak tube18 is a predetermined function of the volume to be filled. In thisparticular embodiment, the input pressure of the fuel gas supplied fromthe tank 19 may be regulated to be of the order of a few pounds persquare inch so that the gas will ow from the leak tube 18 to the outersurface in a period of about thirty microseconds. Such rapid ow of thegas is accomplished primarily because the flow is from a substantialpressure into an almost complete vacuum. The average velocity of the gasow through the evacuated chamber from the leak tube 18 is about fourtimes sonic velocity for the conditions specified herein. With thisvelocity, the sweeping field 13 is utilized once each thirtymicroseconds to prevent interaction between gas particles.

Because of the fact that in the present invention the housing 10 isoperated to be swept clean each time gas particles first reach the innersurface of the housing 10, there is a minimum interaction betweenindividual particles, and minimum inherent longitudinal velocity of thegas. Moreover, the pressure (or gas density) is controlled by the rateof leak of gas from the leak tube 18. By controlling the pressure andtemperature of the leak tube 18 the rate of leak of gas is maintained ata level which will provide a particle density of about 1018 particlesper cubic centimeter. a

In an apparatus of this type some diiculty could be encountered inattaining a predetermined equilibrium temperature of the system. Thisproblem arises for the reason that the rate of the escape of the gas outof the leak tube is a function of the tubes temperature, the tubestemperature in turn is a function of the energy of the ionizingelectro-magnetic wave which in turn increases upon the increase of thegas density within the plasma chamber. Thus can be seen that there is ineffect a chain reaction among the components whereby when more heat isgenerated more gas is released which in turn increases the energy of thewave, which in turn increases the temperature of the leak tube and soon. Such cumu- 3 lative operation is controllable by several rathersimple expedients. In a most direct approach, we can provide for heatexchange cooling of the leak tube 18. However, in a somewhat simplerapproach, we prefer to limit the power available to the field winding12. Although-a direct power limit is feasible, we presently prefer tolimit the frequency of the pulses as a function of the temperature ofthe leak tube 18. In order to accomplish this frequency control, athermocouple 24 senses the temperature of the leak tube 18 and providesa signal to limit the frequency of pulses from the power supply 11 byany conventional means as well as to control the energization of thefilament 22. By limiting the rate of the sweeping field 13 theinteraction of the gas particles effectively reduces the leak rate andthus limits the overall energy expended by the power supply in drivingthe sweeping field 13.

Sweeping of ions at the high velocity contemplated results in thesubstantial energy transfer. In fact, the temperature in the region ofthe sweeping field 13 is of the order of 50,000 K. Such a temperatureassures that all particles are ionized. However, if the engine isoperating substantially continuously the average temperature within thehousing will be of the order of 1,000 C. During such operatingconditions it will be desirable to cool the palladium leak tube 18, amajor portion of which is accomplished automatically by the admission ofcool gas to the inner surfaces thereof. This cool gas receivessubstantial heat energy during its migration through the porous walls ofthe leak tube 18. In the event it is deemed necessary to operate enginesat maximum load for extended periods, it is preferred to admit extremelycold, and in certain arrangements, liquified gas into the leak tube 18.

When an engine of approximately ten centimeters diameter is operatedcontinuously with one pulse during every thirty microseconds and withthe engine chamber approximately one meter long, the thrust will be ofthe order of five milligrams per pulse. Additional thrust is obtainable,usually at some loss in efficiency, by injecting into the system,downstream of the driver section, heavier atoms of various types.

Since the thrust is accomplished by wave propagation, the principlesapplicable to waveguides couplings and coaxial cable couplings willdefine the particular contigurations which are most efficient foradmitting foreign matter. In FIG. 2 we have shown schematically onearrangement for admitting such heavier atoms where the driver section ofthe housing 10 is extended in length by the concentric arrangementtherewith of cylindrical member 27. At the juncture of housing 10 andcylinder 27 there extends at right angles a stub hollow cylinder 26connecting with the interior of the rest of the assembly. The stubmember 26 is so designed to minimize transmission losses by matchingimpedance phenomena well known in this art.

A wide range of materials may be injected into the hot plasma downstreamof the driver section 10 such as waste products, contamination fromexhausted fuel tanks and the like, excess cooling fluids, thrust vectorcontrol fluids, etc. If the injected matter may be easily ionized asweeping magnetic field may be arranged to exert accelerating forcesdirectly on it by energizing coil 28 (FIG. 2). However, usually theprimary accelerating forces on heavy atoms will be due to thebombardment thereon by already accelerated fuel ions.

Referring now to FIG. 3, we have shown a space engine constructed ofthree driving sections 12a, 12b and 12e similar to that illustrated inFIG. l. The sections are combined with a housing 10' arranged withunrestricted flow communication therebetween. In order to pulse thesedriving sections, we have provided these three sweeping eld pulse coils12a, 12b and 12C. If the overall length of the housing 10' is made threemeters, a sweeping voltage may be provided by a three-phase therefore,that our invention is not limited to the pare ticular form shown, and weintend by the appended claims to cover all such modifications which donot depart from the true spirit and scope of our invention.

What is claimed is: 1. A plasma generator and accelerator comprising:elongated housing means having an unrestricted exhaust port at one endthereof; an elongated leak tube arranged around the central axis of saidhousing, said leak tube being of a cross-seetional area of the order ofV of the cross-sectional area of said housing; means for maintaining asupply of ionizable fluid particles of fuel within said leak tube; meansfor controlling the temperature of the leak tube at an elevated level toenhance leakage therethrough of the fuel at a rate creating within saidhousing a density of the order of 101 particles per cubic centimeterwith substantially no reflection from the inner surface of said housing;and means for sequentially propagating electromagnetic sweeping magneticeld from the closed end of said housing to the port thereof, with therate of propagation of the sweeping field being of the order of 105meters per second and with the energy of the sweeping field beingsufficient to assure ionization of substantially all fuel particles. 2.A plasma generator and accelerator comprising: elongated cylindricalhousing means having an unrestricted exhaust port at one end thereof; anelongated leak tube within said housing arranged around the longitudinalaxis of the housing; means for maintaining a pressurized fuel gas insaid leak tube; means for heating said leak tube to enhance leakagetherethrough of gas particles at a rate creating within the housing adensity of the order of 10lo particles per cubic centimeter; means forrepeatedly propagating an electromagnetic wave from the closed end ofsaid housing to the port, with the rate of propagation of the sweepingmagnetic field being of the order of 105 meters per second and with theenergy of the sweeping eld being sufficient to assure ionization ofsubstantially all fuel particles; and means for controlling thetemperature of said leak tube. 3. A plasma generator and acceleratorcomprising: elongated housing means having an unrestricted exhaust portat one end thereof; an elongated leak tube arranged along the major axisof said housing, said leak tube being of a crosssectional area of theorder of V100 of the crosssectional area of said housing; means formaintaining a supply of ionizable gas particles within said leak tube;means for controlling the temperature of the leak tube at an elevatedlevel to enhance leakage therethrough of the gas at a rate creatingwithin said housing a density of the order of 1015 particles per cubiccentimeter with substantially no reflection from the inner surface ofsaid housing; and means for pulsing an electromagnetic sweeping magneticeld from the closed end of said housing to the port thereof, with therate of propagation of the sweeping field being of the order of 105meters per second and with the energy of the sweeping field beingsutiicient to assure ionization of substantially all fuel particles,said leak tube containing ferrite particles for shaping the sweepingfield.

4. A plasma generator and accelerator comprising: elongated cylindricalhousing means having an unrestricted exhaust port at only one endthereof; an elongated leak tube within said housing and arranged aroundthe longitudinal axis thereof; means for maintaining a pressurizedlight-gas fuel in said leak tube;

means for heating said leak tube to enhance leakage therethrough of gasparticles at a rate creating within the housing a density of the orderof le particles per cubic centimeter;

means for repeatedly propagating an electromagnetic wave from the closedend of said housing to the port, with the rate of propagation of thesweeping magnetic field being of the order of 105 meters per second andwith the energy of the sweeping field being sufficient to assureionization of substantialy all fuel particles; and

impedance matched means for inserting particles heavier than thelight-gas fuel into the accelerated ion flow downstream of the port.

5. A space engine comprising:

an elongated cylindrical driving section having an unrestricted exhaustport at only one end thereof;

an elongated leak tube within said section and arranged around thelongitudinal axis thereof;

means for maintaining a pressurized light-gas fuel in said leak tube;

means for controlling the temperature of said leak tube to control therate of leakage of gas therethrough;

pulse coil means for propagating an electromagnetic wave from the closedend of said section to the port with energy sufficient to assureionization of substantially all fuel particles in said section; and

impedance matched means coupled downstream of said section for insertingparticles heavier than the light-gas fuel into the accelerated ion ow toincrease the thrust per pulse.

6. A space engine comprising:

an elongated cylindrical driving section having an unrestricted exhaustport at one end thereof;

an elongated leak tube centrally within said section, said leak tubecontaining a light-gas fuel;

means for controlling the temperature of said leak tube to control therate of leakage of gas therethrough; and

pulse coil means for propagating an electromagnetic wave from the closedend of said section to the port with energy sufiicient to assureionization of substantially all fuel particles in said section.

7. A plasma generator and accelerator comprising:

a plurality of elongated driving sections being aligned in unrestrictedflow communication with each other and having an unrestricted exhaustport at one end thereof;

an elongated leak tube arranged along the major axis of said pluralityof sections, said leak tube being of a cross-sectional area of the orderof $400 of the crosssectional area of said sections;

means for maintaining a supply of ionizable particles of fuel throughoutsaid leak tube;

means for controlling the temperature of the leak tube to regulateleakage therethrough; and

-sequentially operable pulse coil means for propagating anelectromagnetic sweeping magnetic field through each of said sectionstoward the port, with the rate of propagation of the sweeping fieldbeing of the order of 105 meters per second and continuous throughoutall of said sections and with the energy of the sweeping field beingsufiicient to assure ionization of substantially all fuel particles,said leak tube oitaining ferrite particles for shaping the sweeping 8. Aspace engine comprising:

a plurality of elongated driving sections aligned to provideunrestricted flow communication therebetween and having an unrestrictedexhaust port at one end thereof;

an elongated leak tube arranged along the major axis of said pluralityof sections;

means for maintaining a supply of ionizable gas fuel throughout saidleak tube;

sequentially operable pulse coil means for propagating anelectromagnetic sweeping magnetic field through each of said sectionstoward the port, which sweeping field is substantially continuousthroughout all of said sections and with the energy of the sweepingfield being sufiicient to assure ionization of substantially all fuelparticles; and

impedance matched means coupled downstream of the port for insertingparticles heavier than the gas fuel particles into the accelerated ionfiow.

9. A plasma accelerator comprising:

an elongated driving section having an exhaust port at one end thereof;

an elongated leak tube centrally located within the driving section, theperiphery of said leak tube having a porosity sufficient to provide forthe escape therethrough of an ionizable gas fuel whereby the fuel isuniformly supplied to the driving section along substantially its fulllength;

a supply of fuel communicating with the leak tube; and,

pulse coil means propagating an electromagnetic field sequentiallysweeping the length of the driving section, the energy of said fieldbeing sufficient to ionize substantially all the fuel within the drivingsection and to accelerate and expel the ionized fuel through the exhaustport.

l0. A plasma generator and accelerator comprising:

l a driving section having an exhaust port;

leak tube centrally located within the driving section, the walls ofsaid leak tube having a porosity sufficient to provide for the escapetherethrough of an ionizable gas fuel whereby the fuel is uniformlysupplied throughout the driving section;

a supply of fuel communicating with the leak tube;

and

pulse coil means propagating an electromagnetic field sequentiallysweeping the driving section, the energy of said field being sufficientto ionize substantially all the fuel within the driving section and toaccelerate and expel the ionized fuel through the exhaust port.

References Cited in the file of this patent UNITED STATES PATENTS2,578,009 Linder Dec. l1, 1951 2,826,708 Foster Mar. 11, 1958 3,069,344Post et al. Dec. 18, 1962 OTHER REFERENCES Plasma Acceleration (Kash),published by Stanford University Press (Stanford, Calif), 1960 (pages60-72 relied on).

Propulsion Systems for Space Flight (Corliss), published by McGraw-HillBook Co. Inc. (New York), 1960 (pages 2l6 and 220-225),

5. A SPACE ENGINE COMPRISING: AN ELONGATED CYLINDRICAL DRIVING SECTIONHAVING AN UNRESTRICTED EXHAUST PORT AT ONLY ONE END THEREOF; ANELONGATED LEAK TUBE WITHIN SAID SECTION AND ARRANGED AROUND THELONGITUDINAL AXIS THEREOF; MEANS FOR MAINTAINING A PRESSURIZED LIGHT-GASFUEL IN SAID LEAK TUBE; MEANS FOR CONTROLLING THE TEMPERATURE OF SAIDLEAK TUBE TO CONTROL THE RATE OF LEAKAGE OF GAS THERETHROUGH; PULSE COILMEANS FOR PROPAGATING AN ELECTROMAGNETIC WAVE FROM THE CLOSED END OFSAID SECTION TO THE PORT WITH ENERGY SUFFICIENT TO ASSURE IONIZATION OFSUBSTANTIALLY ALL FUEL PARTICLES IN SAID SECTION; AND IMPEDANCE MATCHEDMEANS COUPLED DOWNSTREAM OF SAID SECTION FOR INSERTING PARTICLES HEAVIERTHAN THE LIGHT-GAS FUEL INTO THE ACCELERATED ION FLOW TO INCREASE THETHRUST PER PULSE.